This invention relates to a method of manufacturing a semiconductor device. More particularly, it relates to improvements in a method of bonding a metallic interconnection to a semiconductor region through an opening in an oxide film deposited on the semiconductor region.
As an integrated circuit becomes higher in density and larger in scale, the production of acceptable circuits becomes lower. In this regard, the problem of disconnection especially during the formation of a metallic interconnection is a serious factor. The metallic interconnection is usually made of aluminum (Al), and is formed in such a way that after depositing aluminum by vacuum evaporation or sputtering, it is etched into a required pattern by photolithography. At this time, it often occurs that the aluminum is not deposited in a film of uniform thickness at a stepped part of the pattern. In addition, the deposited aluminum is sometimes lost at the stepped part during the course of forming the pattern. Since, in general, these difficulties are more serious the larger the level difference of the stepped part, there has been proposed that the level difference of a stepped part be reduced and that a stepped part having a gentle slope be formed.
Now, a prior-art method will be described using a P-channel Al-gate MOS FET as an example. FIGS. 1(a) to 1(c) are vertical sectional views which show a part of a semiconductor wafer during the essential steps of a prior-art method of forming openings for contact and metallic interconnections.
As shown in FIG. 1(a), a silicon oxide film (SiO.sub.2 film) 2 is formed on a selected area of a silicon single-crystal substrate 1 of the N-conductivity type. Thereafter, the subtrate is heated in an atmosphere containing diborane (B.sub.2 H.sub.6), to diffuse boron thereinto and to form source and drain regions of the P-conductivity type 3. Subsequently, as shown in FIG. (b), a thick SiO.sub.2 film 4 is formed on the semiconductor substrate 1 after removing the SiO.sub.2 film 2. Further, openings 5 for bonding metallic interconnections to the source and drain regions 3 and an opening 6 for forming a gate insulating film and for connecting an Al interconnection with the gate insulating film are provided in the SiO.sub.2 film 4 by the photolithographic etching. Subsequently, as shown in FIG. 1(c), the gate insulating film 7 is formed on that part of the semiconductor substrate 1 which is exposed through the opening 6. Thereafter, Al is deposited onto those parts of the source and drain regions 3 which are exposed through the openings 5, onto the gate insulating film 7 and onto the SiO.sub.2 film 4, and it is patterned into a required shape so as to form the metallic interconnections 8. Thus, the semiconductor wafer of the P-channel Al-gate MOS FET is completed.
FIGS. 2(a) and 2(b) are enlarged sectional views each showing the state of the Al interconnection 8 at the source or drain region 3.
Ordinarily, those end parts of the SiO.sub.2 film 4 which face the opening for the source or drain region 3 have a steep slope as shown in these figures. As illustrated in FIG. 2(a), the thickness t.sub.1 of the Al interconnection 8 overlying the end part of the SiO.sub.2 film 4 is less than the thickness t.sub.2 of the Al interconnection 8 overlying a flat part of the SiO.sub.2 film 4. In some cases, the Al interconnection 8 becomes broken as illustrated in FIG. 2(b).
FIG. 3 is a plan view in which the portion shown in FIG. 2(a) is seen from the side of the metallic interconnection. Lines A and B in FIG. 3 correspond to points A and B in FIG. 2(a), respectively. Numeral 9 designates a photoresist film. In the step of the selective removal of the deposited Al for forming the pattern of the Al interconnections 8, a constriction often develops in the Al interconnection 8 as shown in FIG. 3.
The breaking and constriction of the Al interconnections 8 are attributed to the fact that the end parts of the SiO.sub.2 film 4 facing the opening have an abrupt inclination. To moderate the effect of such a steep stepped part, there has heretofore been proposed a method in which an SiO.sub.2 film is highly doped with phosphorus to reduce the inclination. With this method, however, it is difficult to obtain a small opening. Another disadvantage is that, since a high-temperature treatment at above 1,050.degree. C. is required, the diffusion of the impurity proceeds rapidly.